| 1. |
- Fan, Xuge, et al.
(author)
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Deformation Behavior and Mechanical Properties of Suspended Double-Layer Graphene Ribbons Induced by Large Atomic Force Microscopy Indentation Forces
- 2022
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In: Advanced Engineering Materials. - : Wiley. - 1438-1656 .- 1527-2648. ; 24:3, s. 2100826-
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Journal article (peer-reviewed)abstract
- Atomic force microscopy (AFM) indentation experiments are commonly used to study the mechanical properties of graphene, such as Young's modulus and strength. However, applied AFM indentation forces on suspended graphene beams or ribbons are typically limited to several tens of nanonewtons due to the extreme thinness of graphene and their sensitivity to damage caused by the AFM tip indentation. Herein, graphene ribbons with a Si mass attached to their center position are employed, allowing us to introduce an unprecedented, wide range of AFM indentation forces (0–6800 nN) to graphene ribbons before the graphene ribbons are ruptured. It is found that the Young's modulus of double-layer graphene ribbons decreases as the applied AFM indentation force is larger than ≈3000 nN, which indicates that the stiffness of double-layer graphene ribbons remains constant before exposing them to AFM indentation forces larger than ≈3000 nN.
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| 2. |
- Fan, Xuge, et al.
(author)
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Direct observation of grain boundaries in graphene through vapor hydrofluoric acid (VHF) exposure
- 2018
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In: Science Advances. - : American Association for the Advancement of Science. - 2375-2548. ; 4:5
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Journal article (peer-reviewed)abstract
- The shape and density of grain boundary defects in graphene strongly influence its electrical, mechanical, and chemical properties. However, it is difficult and elaborate to gain information about the large-area distribution of grain boundary defects in graphene. An approach is presented that allows fast visualization of the large-area distribution of grain boundary–based line defects in chemical vapor deposition graphene after transferring graphene from the original copper substrate to a silicon dioxide surface. The approach is based on exposing graphene to vapor hydrofluoric acid (VHF), causing partial etching of the silicon dioxide underneath the graphene as VHF diffuses through graphene defects. The defects can then be identified using optical microscopy, scanning electron microscopy, or Raman spectroscopy. The methodology enables simple evaluation of the grain sizes in polycrystalline graphene and can therefore be a valuable procedure for optimizing graphene synthesis processes.
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| 3. |
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| 4. |
- Fan, Xuge, et al.
(author)
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Graphene ribbons with suspended masses as transducers in ultra-small nanoelectromechanical accelerometers
- 2019
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In: Nature Electronics. - : Nature Publishing Group. - 2520-1131. ; 2:9, s. 394-404
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Journal article (peer-reviewed)abstract
- Nanoelectromechanical system (NEMS) sensors and actuators could be of use in the development of next-generation mobile, wearable and implantable devices. However, these NEMS devices require transducers that are ultra-small, sensitive and can be fabricated at low cost. Here, we show that suspended double-layer graphene ribbons with attached silicon proof masses can be used as combined spring–mass and piezoresistive transducers. The transducers, which are created using processes that are compatible with large-scale semiconductor manufacturing technologies, can yield NEMS accelerometers that occupy at least two orders of magnitude smaller die area than conventional state-of-the-art silicon accelerometers. With our devices, we also extract the Young’s modulus values of double-layer graphene and show that the graphene ribbons have significant built-in stresses.
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| 5. |
- Fan, Xuge, 1985-, et al.
(author)
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Humidity and CO2 gas sensing properties of double-layer graphene
- 2018
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In: Carbon. - Netherlands : Elsevier. - 0008-6223 .- 1873-3891. ; 127, s. 576-587
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Journal article (peer-reviewed)abstract
- Graphene has interesting gas sensing properties with strong responses of the graphene resistance when exposed to gases. However, the resistance response of double-layer graphene when exposed to humidity and gasses has not yet been characterized and understood. In this paper we study the resistance response of double-layer graphene when exposed to humidity and CO2, respectively. The measured response and recovery times of the graphene resistance to humidity are on the order of several hundred milliseconds. For relative humidity levels of less than ~ 3% RH, the resistance of double-layer graphene is not significantly influenced by the humidity variation. We use such a low humidity atmosphere to investigate the resistance response of double-layer graphene that is exposed to pure CO2 gas, showing a consistent response and recovery behaviour. The resistance of the double-layer graphene decreases linearly with increase of the concentration of pure CO2 gas. Density functional theory simulations indicate that double-layer graphene has a weaker gas response compared to single-layer graphene, which is in agreement with our experimental data. Our investigations contribute to improved understanding of the humidity and CO2 gas sensing properties of double-layer graphene which is important for realizing viable graphene-based gas sensors in the future.
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| 6. |
- Fan, Xuge
(author)
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Integration of graphene into MEMS and NEMS for sensing applications
- 2018
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Doctoral thesis (other academic/artistic)abstract
- This thesis presents a novel approach to integrate chemical vapor deposition (CVD) graphene into silicon micro- and nanoelectromechanical systems (MEMS/NEMS) to fabricate different graphene based MEMS/NEMS structures and explore mechanical properties of graphene as well as their applications such as acceleration sensing, humidity sensing and CO2 sensing. The thesis also presents a novel method of characterization of CVD graphene grain boundary based defects. The first section of this thesis presents a robust, scalable, flexible route to integrate double-layer graphene membranes to a silicon substrate so that large silicon masses are suspended by graphene membranes. In the second section, doubly-clamped suspended graphene beams with attached silicon masses are fabricated and used as model systems for studying the mechanical properties of graphene and transducer elements for NEMS resonators and extremely small accelerometers, occupying die areas that are at least two orders of magnitude smaller than the die areas occupied by the most compact state-of-the-art silicon accelerometers. An averaged Young’s modulus of double-layer graphene of ~0.22 TPa and non-negligible built-in stresses of the order of 200-400 MPa in the suspended graphene beams are extracted, using analytical and FEA models. In addition, fully clamped suspended graphene membranes with attached proof masses are also realized, which are used for acceleration sensing.In the third section, CO2 sensing of single-layer graphene and the cross-sensitivity between CO2 and humidity are shown. The cross-sensitivity of CO2 is negligible at typical CO2 concentrations present in air. The properties of double-layer graphene when exposed to humidity and CO2 have been characterized, with similarly fast response and recovery behaviour but weak resistance responses, compared to single layer graphene.In the fourth section, a fast and simple method for large-area visualization of grain boundaries in CVD graphene transferred to a SiO2 surface is demonstrated. The method only requires vapor hydrofluoric acid (VHF)-etching and optical microscope inspection and therefore could be useful to speed up the process of developing large-scale high quality graphene synthesis, and can also be used for analysis of the influence of grain boundaries on the properties of emerging graphene devices that utilize CVD graphene patches placed on a SiO2 substrate.
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| 7. |
- Fan, Xuge, et al.
(author)
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Manufacture and characterization of graphene membranes with suspended silicon proof masses for MEMS and NEMS applications
- 2020
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In: MICROSYSTEMS & NANOENGINEERING. - : NATURE PUBLISHING GROUP. - 2055-7434. ; 6:1
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Journal article (peer-reviewed)abstract
- Graphene's unparalleled strength, chemical stability, ultimate surface-to-volume ratio and excellent electronic properties make it an ideal candidate as a material for membranes in micro- and nanoelectromechanical systems (MEMS and NEMS). However, the integration of graphene into MEMS or NEMS devices and suspended structures such as proof masses on graphene membranes raises several technological challenges, including collapse and rupture of the graphene. We have developed a robust route for realizing membranes made of double-layer CVD graphene and suspending large silicon proof masses on membranes with high yields. We have demonstrated the manufacture of square graphene membranes with side lengths from 7 mu m to 110 mu m, and suspended proof masses consisting of solid silicon cubes that are from 5 mu mx5 mu mx16.4 mu m to 100 mu mx100 mu mx16.4 mu m in size. Our approach is compatible with wafer-scale MEMS and semiconductor manufacturing technologies, and the manufacturing yields of the graphene membranes with suspended proof masses were >90%, with >70% of the graphene membranes having >90% graphene area without visible defects. The measured resonance frequencies of the realized structures ranged from tens to hundreds of kHz, with quality factors ranging from 63 to 148. The graphene membranes with suspended proof masses were extremely robust, and were able to withstand indentation forces from an atomic force microscope (AFM) tip of up to 7000nN. The proposed approach for the reliable and large-scale manufacture of graphene membranes with suspended proof masses will enable the development and study of innovative NEMS devices with new functionalities and improved performances.
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| 8. |
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| 9. |
- Fan, Xuge, et al.
(author)
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NEMS Sensors Based on Suspended Graphene
- 2021
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In: 2021 IEEE 16Th International Conference On Nano/Micro Engineered And Molecular Systems (Nems). - : Institute of Electrical and Electronics Engineers (IEEE). ; , s. 1169-1172
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Conference paper (peer-reviewed)abstract
- Graphene has exciting potential in nanoelectromechanical system (NEMS) applications due to its unique mechanical and electrical properties as well as its ultimate thinness. In this paper, we discuss the potential of using suspended graphene structures in NEMS sensors and provide an overview of our previous research results on piezoresistive graphene NEMS sensors, including pressure sensors and accelerometers.
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| 10. |
- Fan, Xuge, et al.
(author)
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Rapid and Large-Area Visualization of Grain Boundaries in MoS2 on SiO2 Using Vapor Hydrofluoric Acid
- 2020
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In: ACS Applied Materials and Interfaces. - : American Chemical Society (ACS). - 1944-8244 .- 1944-8252. ; 12:30, s. 34049-34057
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Journal article (peer-reviewed)abstract
- Grain boundaries in two-dimensional (2D) material layers have an impact on their electrical, optoelectronic, and mechanical properties. Therefore, the availability of simple large-area characterization approaches that can directly visualize grains and grain boundaries in 2D materials such as molybdenum disulfide (MoS2) is critical. Previous approaches for visualizing grains and grain boundaries in MoS2 are typically based on atomic resolution microscopy or optical imaging techniques (i.e., Raman spectroscopy or photoluminescence), which are complex or limited to the characterization of small, micrometer-sized areas. Here, we show a simple approach for an efficient large-area visualization of the grain boundaries in continuous chemical vapor-deposited films and domains of MoS2 that are grown on a silicon dioxide (SiO2) substrate. In our approach, the MoS2 layer on a SiO2/Si substrate is exposed to vapor hydrofluoric acid (VHF), resulting in the differential etching of SiO2 at the MoS2 grain boundaries and SiO2 underneath the MoS2 grains as a result of VHF diffusing through the defects in the MoS2 layer at the grain boundaries. The location of the grain boundaries can be seen by the resulting SiO2 pattern using optical microscopy, scanning electron microscopy, or Raman spectroscopy. This method allows for a simple and rapid evaluation of grain sizes in 2D material films over large areas, thereby potentially facilitating the optimization of synthesis processes and advancing applications of 2D materials in science and technology.
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